1. Field of the Invention
This invention relates to a new overageing heat treatment for ductility and weldability improvement of nickel-based superalloys, by means of cyclic overageing techniques. The process of the present invention is applicable to alloys for use in applications where a high material ductility is beneficial, such as in aerospace refurbishment, in applications requiring the improvement of formability in subsequent cold forming and sizing of the alloy, and in applications requiring the improvement of machinability in subsequent machining, cutting, grinding, etc. of the alloy.
2. Description of the Related Art
Repair of aero-engine hot-section components made of nickel-based superalloys are often carried out by welding. These superalloys, especially those advanced grades containing relatively high contents of the strengthening gamma prime phase, are susceptible to cracking during the post-weld heat treatment to regenerate the desired microstructure-dependent properties. As a result, these superalloys are considered to be difficult-to-weld materials. One way to improve the weldability of this group of superalloys is to subject the materials to a pre-weld overageing treatment. This increases the material ductility which, in turn, helps to limit the build-up of residual stress produced by welding, and reduces the susceptibility to post-weld heat treatment (PWHT) cracking of the material.
Conventional pre-weld overageing treatments, such as those currently being used by aero-engine component refurbishment industries, involve heating the material to the solutionizing temperature to dissolve the strengthening gamma prime phase, followed by simple, slow cooling to allow the gamma prime phase to reprecipitate out as coarse particles.
The above overageing scheme involving simple cooling has been found to be effective in improving the weldability of certain nickel-based superalloys, such as Rene 41, as indicated by E. G. Thompson, S. Nunez and M. Prager, 47 Welding Journal 229 (1968). However, this scheme is not very effective for difficult-to-weld, advanced nickel-based superalloys, such as Rene 80. Although blade tip mass build-up by fusion welding has been an acceptable practice in the repair of turbine blades made of Rene 80 nickel-based superalloy, a sizeable rejection rate has often been reported. Furthermore, attempts to repair thermal fatigue cracks at the tip of Rene 80 turbine blades by welding remain unsuccessful to date.